Syria's state news agency says a suicide truck bomb in the central province of Hama has killed three civilians and one security officer.
SANA said the attacker, who covered the bomb with onions, detonated the explosives in the town of Muhrada.
The anti-regime Syrian Observatory for Human Rights said the bomb targeted the local military security headquarters. Both said the dead included two women and a child.
Bombings of security buildings throughout Syria have grown more common as the uprising against President Bashar Assad has become dominated by the rebel insurgency seeking to overthrow his regime.
Activists say more than 17,000 people have been killed since the uprising's start in March 2011, most of them civilians. The government says the dead include more than 4,000 security personnel.